Nerves (sensing types)

[YT2095]

when an area of my hand gets warm or cold I can obviously feel it, are there 2 types of nerves for this (one for hot and one for cold) or is it the same nerve that gives off a different signal to the brain?

 

what "chems" are released and or how does the signal to the brain differ if it`s only an electrical signal?

[Radical Edward]

afaik there are indeed two types, though I am not sure what the structural differences are. Essentially they exist as free nerve endings in the skin, which when stimulated by the appropriate temperature fire a nerve signal. there is no chemical released as such, at least none that travels to the brain, though of course neurotransmitters are released along any synaptic clefts that might be encountered though for the purpose of this analysis they are largely irrelevant. as to how the signal differs, well it's all in the wiring. the different nerves activate different neurons which initiate different cascades of neuron firing within the brain leading to the hot or cold sensation as it were. It's like if you send a signal along line 1 of a parallel port data plug and it activates one part of a program on the computer, and a different data line activates a different part of the program.

[YT2095]

I think I understand, so there`s a nerve for every type of sensation then, like Hot, Cold, Pressure (tactile).

if we just use those 3 nerve types, that`s an awefull lot of "Wiring" per square cm of skin, and then on to the brain, hard to imagine or picture all that goin up the spine or at least the last cervical vertabrae into the brain!

it`s a wonder more things don`t go wrong with us that actualy does.

 

not to worry, it blows my idea out of the water, I was rather hoping it would be purely an electrical signal, perhaps at a certain frequency, that could be fooled into sensations that weren`t true. as either a pain killer or maybe a Virtual Reality suit.

[rakuenso]

however, if there are individual nerves responsible for each type of sensation, are there any documented experiments where an animal can respond to a cold stimuli but not a hot stimula?

[Glider]

Thre isn't a type of nerve for each sensation. There are only a few types of afferent neuron A-alpha, A-beta (large, fast, myelinated fibres), A-delta (smaller, thinly myelinated) and C fibres smallest, slow, non-myelinated).

 

What differentiates their functions is their axon terminals. Differenk kinds of touch receptors mediate different mechanical sensations; vibration, steady indentation, deep pressure and so-on. These are all mediated by speciallised receptors such as Pacinian corpuscles, Meissner's corpuscles, Ruffini corpuscles and Merkel receptors.

 

Only two fibres mediate thermal sensation; A-delta and C fibres. These are primary afferents, also known as nociceptive afferents, becauce they also mediate pain sensation. A-delta fibres are associated with both thermal and mechano-thermal receptors. They mediate warmth, non-noxious cold and mechanical senstation, and are also associated with sharp-pricking pain. C fibres are also accociated with thermal and mechano-thermal receptors. C fibres are polymodal and have a wide dynamic range in that they are not as specific as A-delta fibres in the range of stimulus intensity they respond to. C fibres respond to noxious heat and cold, and are associated with slow, burning or aching pain.

 

So, in answer to the original question "are there 2 types of nerves for this (one for hot and one for cold) or is it the same nerve that gives off a different signal to the brain", there are two types of nerve that mediate thermal sensation. Both mediate hot and cold, but the wide dynamic range C fibres also mediate painful heat and cold.

 

"what "chems" are released....

 

When a receptor is subjected to the stimulus it is specific to it depolarises. The chemical action of depolarisation is more or less the same in every nerve cell. There is a sodium influx raising the potential of the fibre from -70mv to around +40mv. The sodium influx is followed by an efflux of Potassium, and the sodium potassium pump also helps to repolarise the cell.

 

However, C fibres release Substance P as a neurotransmitter. Substance P, once released contributes to oedema and hyperalgesia (increased sensitivity to pain) by causing vasodilation and the release of histamine from mast cells.

 

...and or how does the signal to the brain differ if it`s only an electrical signal?"

 

An action potential is an action potential, and they are all pretty much the same. The reality is that if you cause depolarisation in a cold receptor, then the sensation will be perceived as cold, even if you used electricity to depolarise the cell.

 

This is lablelled-line coding. Activity in a specific sensory fibre will always be perceived as the sensation the fibre is specific to. So, if by some means you could use sound to exite rod & cone cells in the retina, then that sound would be perceived as light. In reality, rod and cone cells will only respond to light, but all sensory receptors can be depolarised by mechanical deformation. If you push the side of your eyeball, you deform a part of the retina and this will be perceived as light.

 

A more common example is eating chilli. The active ingredient in capsaicin, this is a crystalline alkaloid and is neither hot nor cold. However, it does quite specifically target C fibres (it is toxic to them and causes in influx of calcium, which eventually kills the cell. This is why people can adapt to hotter and hotter chilli/curry).

 

So, although capsaicin is neither hot nor cold, it does trigger receptors that mediate hot & cold sensation. It triggeres them a lot, causing high frequency volleys of action potentials, which are perceived as noxious heat, although there is no actual noxious tempterature involved.

[YT2095]

Thanks again

 

I`m not sure I can use that info in any practical way, but nothing learned is wasted.

 

with regards to the action of capsiacin and calcium influx, would adding salt to the chili pepper affect this reaction at all?

 

maybe it`s just a matter of taste, but I find chilis seem to taste better with salt added, and when sugar is used instead, it seems to counter alot of the "Heat".

 

is there any biological mechanism behind this, or is it just a matter of personal taste?

[Glider]

I doubt adding salt makes much of a difference at the molecular level, but I agee, it does seem to improve the taste.

 

Given the nature of capsaicin (a plant alkaloid), the best way to counter the heat is to apply a mild acid. Citric acid works well, lemon, orange or lime, as does tannic acid in tea. A bad thing to do is to drink water, that just puts more capsaicin into solution, spreads it around some more and generally makes things worse.

[YT2095]

actualy capsiacin is only alcohol and oil soluble, a glass off milk or yogurt (riata dip) or even a peice of cheese will disperse the active part quite well, it`ll not neutralise it, but it takes some of the heat away after you swallow.

 

as for the acids, i`ve noticed that when I pickle my chilis they do tend to become alot milder, and as for the salt, I was quite prepared for it to be a matter of personal taste without there being much of a scientific reason behind it, the sugar part intrigues me though?

it almost seems to work as a type of antidote, a bit like adding potatoes or double cream to a food that`s been over salted. could it perhaps be a way of "tricking" the nerves into a counter reaction?

 

one group says one thing, another group says something different, and the mean average of the mixed signals gives a nullifying effect?

[Glider]

Capsaicin "is slightly soluble in water, but very soluble in alcohol, fats, and oils."

(http://www.fiery-foods.com/dave/capsaicin.asp).

 

I don't know about the sugar. I haven't heard of any reason for it, but then, I haven't looked for one either. It's intriguing though. I'll have to try it next time I have a chilli.

[YT2095]

just doing a quick flick trough google, I found this:

 

"A. If you want to keep some of the heat, but moderate it to tolerable levels, this is a good solution. Sugars and acids together can help neutralize the sting of capsaicin. This is something to keep in mind when eating at Asian restaurants, as well: If you want to order one rather hot dish, you can choose a sweet-and-sour to go with it, as a way of toning your palate down."

 

taken from: http://www.exploratorium.edu/cooking/seasoning/dinner_party/printerfriendly.html

 

the only reason I actualy know about this was when I went to the "Good Food" show, and tried an assortment of different pickled chilis (for free!).

the ones made with a sweet vinigar were alot more tame than the same variety in plain vinigar or oil.

 

I`ve no idea if it`s a chemical reaction, or if it tricks the nerves with conflicting data or what?

[Glider]

I have no idea either, but it's a good thing to know.

[YT2095]

just doing my regular research on the humble Chili pepper and came across this site, it`s got everything you wanted to know about them but were afraid to ask

 

http://www.mdidea.com/products/new/new005.html

 

it may come in handy as a reference later on for someone.